If you truly want to solve this you need to figure out completely what your horse's motivation for rearing is. You said she rears during groundwork. What is causing the rearing? Is she becoming frightened? Feeling trapped? Not understanding cues and getting overfaced? Did she start it and then get out of work?
Before you would even think about trying to solve the rearing under saddle, you would first need to solve the problem on the ground. Is it always when you are asking her to move forward? In other words, is she evading work by rearing during ground work and because of this has started to try the trick under saddle?
Pay close attention to her body language during rearing. I have seen horses rear as a cute sort of trick with their ears up and looking at you for your reaction. I have seen horses trying to establish dominance and rearing while facing you in a threatening manner. And I have seen horses rearing with their eyes rolling back in their head because they are panicked.
It is very important to understand a horse's motivation when dealing with a problem behavior.